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I've been dealing with PayPal subscriptions for a little while now, using it to manage quite a complex subscription scheme. Though I encrypt my PayPal buttons, the notifications from PayPal are sent to an unencrypted URL, so I need check with PayPal to make sure the notifications are in fact from them. To do this, one sends back the notification's request parameters, to which PayPal respond with a 'VERIFIED' or 'INVALID' string.